English and Icelandic are the only Germanic languages with both "th" sounds

I supposed that list could be expanded to Faroese, etc. My point is all those are (or were) island languages. Thus, their relative isolation insulated them (pun intended, insula means island in Latin) from the changes underwent by German, Dutch, Swedish, etc.

There is probably something similar at work with Polynesian vs. other Austronesian languages. 

Not much of an insight but figured I'd share it. Arabic also has the same "th" sounds in the letters thaa ث and dhaal ذ, though those sounds are often substituted with daal د (d) and zayn ز (z) respectively. This makes sense as Arabia used to be fairly isolated from the rest of the world.